(1) Field of the Invention
The present invention relates to a modulator, a demodulator, and a modem containing the modulator and the demodulator. The present invention relates, in particular to a modem having provision for modulating and demodulating signals carrying information on conditions of the modem and another modem connected with the modem through an analog transmission line, through a secondary channel which is provided in addition to a main channel. The main channel and the secondary channel are frequency division multiplexed within a frequency band of the analog transmission line.
(2) Description of the Related Art
FIG. 1 is a diagram illustrating an on-line information processing system wherein a plurality of pieces of data terminal equipment are connected through analog transmission lines to a host computer. In FIG. 1, reference numeral 900 denotes a host computer, 901 denotes a communication control processor, 903 denotes a network monitoring apparatus, 911.sub.1 to 911.sub.n each denote a modem on the host computer side, 912.sub.1 to 912.sub.n each denote a modem on the terminal side, 913.sub.1 to 913.sub.n each denote a piece of data terminal equipment, and 914.sub.1 to 914.sub.n each denote an analog transmission line. For example, the host computer 900 and the communication control processor 901 are provided in a main office of a company, and the plurality of pieces of data terminal equipment are respectively positioned in a plurality of branch offices. Digital signals are transmitted between the host computer 900 and the plurality of pieces of data terminal equipment 913.sub.1 to 913.sub.n through the analog transmission lines 914.sub.1 to 914.sub.n. When transmitting the digital signals through the analog transmission lines 914.sub.1 to 914.sub.n, the digital signals are modulated to analog signals by the modulators on transmitter sides, transmitted through the analog transmission lines, and demodulated to digital signals by the demodulators on receiver sides. Each modem 911.sub.1 to 911.sub.n and 914.sub.1 to 914.sub.n has provision for monitoring various conditions thereof, and a secondary channel is provided in addition to a main channel for transmitting the above digital signals.
As mentioned above, the main channel and the secondary channel are frequency division multiplexed within a frequency band of the analog transmission line. FIG. 2 is a diagram illustrating frequency bands allocated for the main channel and the secondary channel within the frequency band of the analog transmission line (0.3 to 3.4 kHz). As indicated in FIG. 2, the frequency band for the secondary channel is allocated on the lower frequency side of the frequency band of the main channel, separated from the frequency band of the main channel. In the example of FIG. 2, the frequency band of the main channel ranges from 600 to 3,000 Hz, and the frequency band of the secondary channel ranges from 311 to 359 Hz. The center frequencies of the frequency bands of the main channel and the secondary channel are 1,800 and 335 Hz, respectively.
The network monitoring apparatus 903 collects information on the conditions monitored by the respective modems 911.sub.1 to 911.sub.n and 914.sub.1 to 914.sub.n through the above secondary channels and the signal lines 915.sub.1 to 915.sub.n. FIG. 3 is a diagram illustrating a connection between the network monitoring apparatus 903 and the plurality of modems 911.sub.1 to 911.sub.n on the side of the host computer 900. As indicated in FIG. 3, the plurality of modems 911.sub.1 to 911.sub.n are connected with the network monitoring apparatus 903 in the form of a bus connection. The signal lines 915.sub.1 to 915.sub.n connecting the network monitoring apparatus 903 with the plurality of modems 911.sub.1 to 911.sub.n are balanced signal lines in accordance with RS485, and the network monitoring apparatus 903 collects the above information by polling the respective modems 911.sub.1 to 911.sub.n. In the communication between the network monitoring apparatus 903 and the plurality of modems 911.sub.1 to 911.sub.n, a start-stop system is used, i.e., a start bit and a stop bit are affixed to each character (comprised of eight bits) in the information transmitted between the network monitoring apparatus 903 and the plurality of modems 911.sub.1 to 911.sub.n.
FIG. 4 is a diagram illustrating a construction of a conventional modem which can be used in the system as indicated in FIG. 1. In FIG. 4, reference numeral 800 denotes a piece of data terminal equipment, 110a to 110i each denote a driver circuit, 40' denotes a main signal modulation unit, 50' denotes a main signal demodulation unit, 60a denotes a secondary signal modulation unit, 60b denotes a secondary signal demodulation unit, 70' denotes a microprocessor unit (MPU) as a command control unit, 41' and 51' each denote a microprocessor unit, 42' and 52' each denote a digital signal processor (DSP), 43' and 53' each denote a digital to analog converter, 11 denotes a signal adder, 90a and 90b each denote a low-pass filter, 90c and 90d each denote a high-pass filter, and 903 denotes the network monitoring apparatus of FIG. 1.
In the construction of FIG. 4, digital signals supplied from the data terminal equipment 800 are modulated to analog passband signals within the frequency band of the main channel in the main signal modulation unit 40' to be supplied to the signal adder 11. A command or data to be transmitted through the analog transmission line is generated by the microprocessor unit 70', modulated to an analog passband signal in the secondary signal modulation unit 60a, and filtered through the low-pass filter 90a to be supplied to the signal adder 11. Analog signals transmitted from the analog transmission line are supplied to the main signal demodulation unit 50' and the low-pass filter 90b. Frequency components corresponding to the main channel in the analog signals pass through the high-pass filter 90d, are converted to digital signals, and demodulated through the digital signal processor 52' and the microprocessor unit 51'. Frequency components corresponding to the secondary channel in the analog signals pass through the low-pass filter 90b, are converted to digital signals, and demodulated through the secondary signal demodulation unit 60b. The demodulated secondary signals are supplied to the microprocessor unit 70'.
Conventionally, the modulation and demodulation of the main signals are performed in accordance with phase shift keying (PSK) or quadrature amplitude modulation (QAM), and the modulation and demodulation of the secondary signals are performed in accordance with frequency shift keying (FSK). The demodulation in accordance with the frequency shift keying (FSK) can be performed without synchronization with a receiving clock signal regenerated from the received signal, while the demodulation in accordance with the phase shift keying (PSK) or the quadrature amplitude modulation (QAM) must be performed in synchronization with the receiving clock. Therefore, the modulation and demodulation of the main signals are performed by using the digital signal processor and the microprocessor unit, and the modulation and demodulation of the secondary signals are performed by using a one chip modem, which is commercially available and realizes the secondary signal modulation unit 60a and the secondary signal demodulation unit 60b.
Since the modulation and demodulation of the main signals and the secondary signals are carried out in separate hardware, the amount of the hardware is increased, and therefore, the size of and the cost for the modem are increased.